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Reaction Timer 


Reaction Timer 

Written By: Charles Piatt 


Multimeter (1) 


Decade Counter chips (4) 
Really you need only 3, but get another 
one in case you damage the others. 
Good sources for some or all of these 
components include RadioShack (retail 
locations and Mouser 
Electronics ( Digi-Key 
( Newark Online 
( and All Electronics 
Corporation (allelectronics. com). 
Timer chips (3) 

Do not get a CMOS or any high- 
precision versions. 
Switches (3) 
LED display (1) 

such as the Kingbright BC56- 1 1EWA. Or 
three numeric LEDs. 
Breadboard (1) 

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resistors (9) 
Capacitors (8) 
Potentiometer (1) 

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Reaction Timer 


DIP IC sockets (3) 
Jumper wires (1) 
Power supply (1) 


Make: Electronics is an electronics primer for the early 21st century. It's written for the 
absolute beginner and all those who've wanted to learn electronics. Those who've wanted to 
build all the cool kits out there, or to try their hand at programming microcontrollers, but 
who've found themselves intimidated by existing books and online resources that seem to be 
written by deep geeks for deep geeks. 

Make: Electronics is written in a fun, clear-spoken, graphical style. It includes 36 
experiments and projects, plus dozens of sidebars on the science, history, and personalities 
behind electronics. And it's brimming with hundreds of photos, illustrations, diagrams, 
schematics, even cartoons, all done by Charles Piatt! 

It was Piatt's beginner electronics guide and 555 timer projects in MAKE Volume 10 that 
made us realize he might be the man to pull off the book we desired. So it's fitting that we've 
chosen this new 555 timer project to present here. 

It occurs midway through the book, as Experiment 18, so it's a bit advanced for the beginner 
(don't worry, the book starts off with very easy fare), but if you follow the instructions 
carefully, you'll be fine. And one of the core lessons of the book is to not be afraid of failure, 
so if it takes you a few tries, that's fine too. 

Be patient and learn from your mistakes. (If you're new to electronics you might want to read 
Piatt's "Your Electronics Workbench" and do the projects in "The Biggest Little Chip," both in 
Volume 10, before tackling this project.) 

We hope you enjoy this peek at Make: Electronics , and pick up a copy for yourself, a friend, 
or a family member. They're probably tired of seeing you having all the geeky fun, but are 
too embarrassed to let you in on their ignorance. We know they're out there. 

When we announced the book on Make: Online , we started getting "confessional" posts from 
readers. One wrote: "Prepare yourselves. You're going to sell one BILLION of these books. 
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Reaction Timer 

This is exactly what I've been looking for, for over a decade." Thanks. We made this book 
for you. (And we'll settle for a million.) 

— Gareth Branwyn 

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Reaction Timer 

Step 1 — Display. 

• Because the 555 timer chip can 
easily run at thousands of cycles 
per second, we can use it to 
measure human reactions. You can 
compete with friends to see who 
has the fastest response — and 
note how your response changes 
depending on your mood, the time 
of day, or how much sleep you got 
last night. 

• Before going any further, I have to 
warn you that this circuit requires a 
lot of wiring, and will only just fit on 
a breadboard that has 63 rows of 
holes. Still, we can build it in a 
series of phases, which should 
help you to detect any wiring errors 
as you go. 

• You can use three separate LED 
numerals for this project, but I 
suggest that you buy the Kingbright 
BC56-11EWA, which contains 
three numerals in one big package. 

• You should be able to plug it into 
your breadboard, straddling the 
center channel. Put it all the way 
down at the bottom of the 
breadboard, as shown in the step 
photo. Don't put any other 
components on the breadboard yet. 

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Reaction Timer 

Step 2 

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Reaction Timer 

• Now set your power supply to 9 volts (or use a 9-volt battery), and apply the negative side 
of it to the row of holes running up the breadboard on the right-hand side. Insert a 1K 
resistor between that negative supply and each of pins 18, 19, and 26 of the Kingbright 
display, which are the "common cathode," meaning the negative connection shared by 
each set of LED segments in the display. 

• The pin numbers of the chip are shown in the first step photo. If you're using 
another model of display, you'll have to consult a data sheet to find which pin(s) are 
designed to receive negative voltage. 

• Switch on the power supply and touch the free end of the positive wire to each row of holes 
serving the display on its left and right sides. You should see each segment light up, as 
shown in the photo in Step 1 . 

• Each numeral from to 9 is represented by a group of these segments. The segments are 
always identified with lowercase letters a through g, as shown in the second step photo. In 
addition, there is often a decimal point, and although we won't be using it, I've identified it 
with the letter h. 

• Check the first step photo showing the Kingbright display, and you'll see I have annotated 
each pin with its function. You can step down the display with the positive wire from your 
power supply, making sure that each pin lights an appropriate segment. 

• Incidentally, this display has two pins, numbered 3 and 26, both labeled to receive negative 
voltage for the first of the digits. 

• Why two pins instead of one? I don't know. You need to use only one, and as this is a 
passive chip, it doesn't matter if you leave the unused one unconnected. Just take care 
not to apply positive voltage to it, which would create a short circuit. 

• A numeric display has no power or intelligence of its own. It's just a bunch of light-emitting 
diodes. It's not much use, really, until we can figure out a way to illuminate the LEDs in 
appropriate groups — which will be the next step. 

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Reaction Timer 

Step 3 — Counting. 

• Fortunately, we have a chip known 
as the 4026, which receives 
pulses, counts them, and creates 
an output designed to work with a 
seven-segment display so that it 
shows numbers 0-9. The only 
problem is that the 4026 is a rather 
old-fashioned CMOS chip 
(meaning, Complementary Metal 
Oxide Semiconductor) and is thus 
sensitive to static electricity. 

• Switch off your power supply and 
connect its wires to the top of the 
breadboard, noting that for this 
experiment, we're going to need 
positive and negative power on 
both sides. See the photo for 
details. If your breadboard doesn't 
already have the columns of holes 
color-coded, I suggest you use 
Sharpie markers to identify them, 
to avoid polarity errors that can fry 
your components. 

• New line.The 4026 counter chip is 
barely powerful enough to drive the 
LEDs in our display when powered 
by 9 volts. Make sure you have the 
chip the right way up, and insert it 
into the breadboard immediately 
above your three-digit display, 
leaving just one row of holes 
between them empty. 

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Reaction Timer 

Step 4 

• The schematic in the first step photo shows how the pins of the 4026 chip should be 
connected. The arrows tell you which pins on the display should be connected with pins on 
the counter. 

• The second step photo shows the "pinouts" (i.e., the functions of each pin) of a 4026 
counter chip. You should compare this with the schematic in the first step photo. 

• Include a tactile switch between the positive supply and pin 1 of the 4026 counter, with a 
10K resistor to keep the input to the 4026 counter negative until the button is pressed. 
Make sure all your positives and negatives are correct, and turn on the power. 

• You should find that when you tap the tactile switch lightly, the counter advances the 
numeric display from through 9 and then begins all over again from 0. You may also find 
that the chip sometimes misinterprets your button-presses, and counts two or even three 
digits at a time. I'll deal with this problem a little later on. 

• The LED segments won't be glowing very brightly, because the 1K series resistors deprive 
them of the power they would really like to receive. Those resistors are necessary to avoid 
overloading the outputs from the counter. 

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Reaction Timer 

Step 5 

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Reaction Timer 

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• Assuming that you succeed in 
getting your counter to drive the 
numeric display, you're ready to 
add two more counters, which will 
control the remaining two 
numerals. The first counter will 
count in ones, the second in tens, 
and the third in hundreds. 

• In the step photo, I've used arrows 
and numbers to tell you which pins 
of the counters should be 
connected to which pins of the 
numeric display. Otherwise, the 
schematic would be a confusing 
tangle of wires crossing each 

• At this point, you can give up in 
dismay at the number of 
connections — but really, using a 
breadboard, it shouldn't take you 
more than half an hour to complete 
this phase of the project. I suggest 
you give it a try, because there's 
something magical about seeing a 
display count from 000 through 999 
"all by itself," and I chose this 
project because it also has a lot of 
instructional value. 

• S1 is attached to the "clock 
disable" pin of IC1, so that when 
you hold down this button, it should 
stop that counter from counting. 
Because IC1 controls IC2, and IC2 
controls IC3, if you freeze IC1, the 
other two will have to wait for it to 
resume. Therefore you won't need 

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Reaction Timer 

to make use of their "clock disable" 

• S2 is connected to the "reset" pins 
of all three counters, so that when 
you hold down this button, it should 
set them all to zero. 

• S3 sends positive pulses manually 
to the "clock input" pin of the first 

• S1 , S2, and S3 are all wired in 
parallel with 1K resistors 
connected to the negative side of 
the power supply. The idea is that 
when the buttons are not being 
pressed, the "pull-down" resistors 
keep the pins near ground (zero) 
voltage. When you press one of the 
buttons, it connects positive 
voltage directly to the chip, and 
easily overwhelms the negative 
voltage. This way, the pins remain 
either in a definitely positive or 
definitely negative state. 

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Reaction Timer 

Step 6 

• If you disconnect one of these pull-down resistors you are likely to see the numeric display 
"flutter" erratically. (The numeric display chip has some unconnected pins, but this won't 
cause any problem, because it is a passive chip that is just a collection of LED segments.) 

• Always connect input pins of a CMOS chip so that they are either positive or 

• I suggest that you connect all the wires shown in the schematic first. Then cut lengths of 
22-gauge wire to join the remaining pins of the sockets from IC1, IC2, and IC3 to IC4. 

• Switch on the power and press S2. You'll see three zeros in your numeric display. 

• Each time you press S3, the count should advance by 1 . If you press S2, the count should 
reset to three zeros. If you hold down S1 while you press S3 repeatedly, the counters 
should remain frozen, ignoring the pulses from S3. 

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Reaction Timer 

Step 7 — Pulse generation. 

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Reaction Timer 

To Pin 1 

of 4026 




• A 555 timer is ideal for creating a 
stream of pulses that drive a 
counter chip. The image shows 
how to connect these chips to the 
positive and negative rails on your 
breadboard. Also I'm showing the 
connection between pins 2 and 6 in 
the way that you're most likely to 
make it, via a wire that loops over 
the top of the chip. 

• For the current experiment, I'm 
suggesting initial component values 
that will generate only four pulses 
per second. Any faster than that, 
and you won't be able to verify that 
your counters are counting 

• Install IC5 and its associated 
components on your breadboard 
immediately above IC1. Don't leave 
any gap between the chips. 
Disconnect S3 and R3 and connect 
a wire directly between pin 3 
(output) of IC5 and pin 1 (clock) of 
IC1 , the topmost counter. 

• Power up again, and you should 
see the digits advancing rapidly in 
a smooth, regular fashion. Press 
S1 , and while you hold it, the count 
should freeze. Release S1 and the 
count will resume. Press S2 and 
the counter should reset, even if 
you are pressing S1 at the same 

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Reaction Timer 

Step 8 — Refinements. 

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Reaction Timer 

• Now it's time to remember that what we really want this circuit to do is test a person's 
reflexes. When the user starts it, we want an initial delay, followed by a signal — probably 
an LED that comes on. The user responds to the signal by pressing a button as quickly as 
possible. During the time it takes for the person to respond, the counter will count milli- 
seconds. When the person presses the button, the counter will stop. The display then 
remains frozen indefinitely, displaying the number of pulses that were counted before the 
person was able to react. 

• How to arrange this? I think we need a flip-flop. When the flip-flop gets a signal, it starts 
the counter running — and keeps it running. When the flip-flop gets another signal (from 
the user pressing a button), it stops the counter running, and keeps it stopped. 

• How do we build this flip-flop? Believe it or not, we can use yet another 555 timer, in a new 
manner known as bistable mode. 

• In bistable mode, the 555 has turned into one big flip-flop. To avoid any uncertainty, we 
keep pins 2 and 4 normally positive via pull-up resistors, but negative pulses on those pins 
can overwhelm them when we want to flip the 555 into its opposite state. 

• The schematic for running a 555 timer in bistable mode, controlled by two pushbuttons, is 
shown in the first step photo. You can add this above your existing circuit. Because you're 
going to attach the output from IC6 to pin 2 of IC1 , the topmost counter, you can 
disconnect S1 and R1 from that pin. See second step photo. 

• Now, power up the circuit again. You should find that it counts in the same way as before, 
but when you press S4, it freezes. This is because your bistable 555 timer is sending its 
positive output to the "clock disable" pin on the counter. The counter is still receiving a 
stream of pulses from the astable 555 timer, but as long as pin 2 is positive on the 
counter, the counter simply ignores the pulses. 

• Now press S5, which flips your bistable 555 back to delivering a negative output, at which 
point the count resumes. We're getting close to a final working circuit here. We can reset 
the count to zero (with S3), start the count (with S5), and wait for the user to stop the 
count (with S4). The only thing missing is a way to start the count unexpectedly. 

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Reaction Timer 

Step 9 — The delay. 

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Reaction Timer 

• Suppose we set up yet another 555 
in mono-stable mode. Trigger its 
pin 2 with a negative pulse, and the 
timer delivers a positive output that 
lasts for, say, 4 seconds. At the 
end of that time, its output goes 
back to being negative. Maybe we 
can hook that positive-to-negative 
transition to pin 4 of IC6. We can 
use this instead of switch S5, 
which you were pressing 
previously to start the count. 

• Check the final schematic to the 
left (repeated for your 
convenience), which adds another 
555 timer, IC7 above IC6. When 
the output from IC7 goes from 
positive to negative, it will trigger 
the reset of IC6, flipping its output 
negative, which allows the count to 
begin. So IC7 has taken the place 
of the start switch, S4. You can get 
rid of S4, but keep the pull-up 
resistor, R9, so that the reset of 
IC6 remains positive the rest of the 

• This arrangement works because I 
have used a capacitor, C4, to 
connect the output of IC7 to the 
reset of IC6. The capacitor 
communicates the sudden change 
from positive to negative, but the 
rest of the time it blocks the steady 
voltage from IC7 so that it won't 
interfere with IC6. 

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Reaction Timer 

Step 10 

11 p^.® w v- 




• The final schematic shows the three 555 timers all linked together, as you should insert 
them above the topmost counter, IC1. I also added an LED to signal the user. The second 
picture is a photograph of my working model of the circuit. 

• Because this circuit is complicated, I'll summarize the sequence of events when it's 
working. Refer to the final schematic while following these steps: 

• User presses Start Delay button S4, which triggers IC7. 

• IC7 output goes high for a few seconds while C5 charges. 

• IC7 output drops back low. 

• IC7 communicates a pulse of low voltage through C4 to IC6, pin 4. 

• IC6 output flips to low and flops there. 

• Low output from IC6 sinks current through an LED and lights it. 

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Reaction Timer 

Step 11 

• Sequence of events cont'd: 

• Low output from IC6 also goes to pin 2 of IC1 . 

• Low voltage on pin 2 of IC1 allows IC1 to start counting. 

• User presses S3, the "stop" button. 

• S3 connects pin 2 of IC6 to ground. 

• IC6 output flips to high and flops there. 

• High output from IC6 turns off the LED. 

• High output from IC6 also goes to pin 2 of IC1 . 

Step 12 

• Sequence of events cont'd. 

• High voltage on pin 2 of IC1 
stops it from counting. 

• After assessing the result, user 
presses S2. 

• S2 applies positive voltage to pin 
15 of IC1, IC2, andlC3. 

• Positive voltage resets counters 
to zero. 

• The user can now try again. 

• Meanwhile, IC5 is running 

• In case you find a block diagram 
easier to understand, I've 
included that, too. 

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Reaction Timer 

Step 13 — Use the reflex tester. 

• At this point, you should be able to fully test the circuit. When you first switch it on, it will 
start counting, which is slightly annoying, but easily fixed. Press S3 to stop the count. 
Press S2 to reset to zero. 

• Now press S4. Nothing seems to happen — but that's the whole idea. The delay cycle has 
begun in stealth mode. After a few seconds, the delay cycle ends, and the LED lights up. 
Simultaneously, the count begins. As quickly as possible, the user presses S3 to stop the 
count. The numerals freeze, showing how much time elapsed. 

• There's only one problem — the system hasn't yet been calibrated. It's still running in 
slow-motion mode. You need to change the resistor and capacitor attached to IC5 to make 
it generate 1 ,000 pulses per second instead of just three or four. 

• Substitute a 10K trimmer potentiometer for R8 and a 1 F capacitor for C2. This 
combination will generate about 690 pulses per second when the trimmer is presenting 
maximum resistance. When you turn the trimmer down to decrease its resistance, 
somewhere around its halfway mark the timer will be running at 1,000 pulses per second. 

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Reaction Timer 

Step 14 

• How will you know exactly where this point is? Ideally, you'd attach an oscilloscope probe 
to the output from IC5. But, most likely you don't have an oscilloscope, so here are a 
couple other suggestions. 

• First remove the 1 F capacitor at C2 and substitute a 10 F capacitor. Because you are 
multiplying the capacitance by 10, you'll reduce the speed by 10. The leftmost digit in your 
display should now count in seconds, reaching 9 and rolling over to every 10 seconds. 
You can adjust your trimmer potentiometer while timing the display with a stopwatch. 
When you have it right, remove the 10 F capacitor and replace the 1 F capacitor at C2. 

• The only problem is, the values of capacitors may be off by as much as 10%. If you want 
to fine-tune your reflex timer, you can proceed as follows. Disconnect the wire from pin 5 
of IC3, and substitute an LED with a 1K series resistor between pin 5 and ground. Pin 5 is 
the "carry" pin, which will emit a positive pulse whenever IC3 counts up to 9 and rolls over 
to start at again. Because IC3 is counting tenths of a second, you want its carry output 
to occur once per second. 

• Now run the circuit for a full minute, using your stopwatch to see if the flashing LED drifts 
gradually faster or slower than once per second. If you have a camcorder that has a time 
display in its viewfinder, you can use that to observe the LED. 

• If the LED flashes too briefly to be easily visible, you can run a wire from pin 5 to another 
555 timer that's set up in monostable mode to create an output lasting for around 1/10 of a 
second. The output from that timer can drive an LED. 


It goes without saying that anytime you finish a project, you see some opportunities to improve 
it. Here are some suggestions: 

No counting at power-up. It would be nice if the circuit begins in its "ready" state, rather than already counting. To achieve 
this you need to send a negative pulse to pin 2 of IC6, and maybe a positive pulse to pin 15 of IC1 . Maybe an extra 555 
timer could do this. I'm going to leave you to experiment with it. Audible feedback when pressing the Start button. Currently, 
there's no confirmation that the Start button has done anything. All you need to do is buy a piezoelectric beeper and wire it 
between the right-hand side of the Start button and the positive side of the power supply. A random delay interval before the 
count begins. Making electronic components behave randomly is very difficult, but one way to do it would be to require the 
user to hold his finger on a couple of metal contacts. The skin resistance of the finger would substitute for R1 1 . Because the 
finger pressure wouldn't be exactly the same each time, the delay would vary. You'd have to adjust the value of C5. 

Summing Up 

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Reaction Timer 

This project demonstrated how a counter chip can be controlled, how counter chips can be 
chained together, and three different functions for 555 timers. It also showed you how chips can 
communicate with each other, and introduced you to the business of calibrating a circuit after 
you've finished building it. 

Naturally, if you want to get some practical use from the circuit, you should build it into an 
enclosure with heavier-duty pushbuttons — especially the button that stops the count. You'll find 
that when people's reflexes are being tested, they are liable to hit the Stop button quite hard. 

This project first appeared in MAKE Volume 21 . page 96. 

last generated on 2012-10-31 11:45:49 PM. 

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